As a simple linear operator usable for improving a blur image, there is an unsharp masking (UM) technique (refer to A. K. Jain (1989), “Fundamentals of Digital Image Processing,” Prentice Hall, Inc., Englewood Cliffs, N.J. (Non-Patent Document 1)). The unsharp masking technique takes advantage of a characteristic of human visual system, called Mach band effect. This characteristic brings a visual phenomenon that perceived luminance of regions adjacent to each other depends on sharpness of a transition portion. Taking advantage of this characteristic, the sharpness of an image can be improved by introducing a larger number of apparent changes between image regions. The basic idea of unsharp masking technique is to subtract, from an input signal, a signal obtained by filtering the input signal through a low-pass filter.
FIG. 6 is a block diagram showing an example of a configuration of a conventional unsharp masking device for performing the above-described unsharp masking technique. An unsharp masking device 11 includes: a smoothing means 12 for smoothing luminance of an input image and then supplying the smoothed image to a subtraction means 13; the subtraction means 13 for calculating a difference image by subtracting the smoothed image from the luminance of the input image, and then supplying the difference image to a multiplication means 14; the multiplication means 14 for multiplying the difference image by a constant number and then supplying the multiplication result to an addition means 15; and the addition means 15 for adding the multiplication result to the luminance of the input image and then outputting luminance of an output image.
Various types of edge enhancement processing using the unsharp masking technique exist. Examples of the edge enhancement processing include sharpening by use of Laplacian (refer to pp. 58 to 59, and pp. 72 to 73 in TSUCHIYA, Yutaka and FUKADA, Youji, “Image Processing,” the Institute of Image Information and Television Engineers Reference Book Series, Corona Publishing Co., Ltd, ISBN4-339-01070-7 (Non-Patent Document 2)), and crispening (disclosed in FIGS. 1 and 2 of Japanese Patent Application Publication No. Sho 52-266116 (Patent Document 1)).
Patent Document 1: Japanese Patent Application Publication No. Sho 52-266116 Non-Patent Document 1: A. K. Jain (1989), “Fundamentals of Digital Image Processing,” Prentice Hall, Inc., Englewood Cliffs, N.J. Non-Patent Document 2: TSUCHIYA, Yutaka and FUKADA, Youji, “Image Processing,” the Institute of Image Information and Television Engineers Reference Book Series, Corona Publishing Co., Ltd, ISBN4-339-01070-7